Many cosmetic formulations are currently intended to be applied to skin: this is an intrinsic feature of beauty products, make-up products and body-care products. It is sought, very naturally, to thicken such formulations in a pH range matching that of skin, i.e. at values of between 5 and 7, and preferentially between 5 and 6.5, and very preferentially between 5.5 and 6.
There are a number of technical solutions to this problem, which may be classified into 4 categories: use of polymers of higher molecular weights and in powder form, the technique called “back-acid”, which relies on acrylic polymers in direct emulsion of polymer particles in water, use of other polymers, again in the form of direct emulsions, and finally the use of reverse emulsions.
In the first category, one may cite document EP 1 138 703 A1, which describes a cosmetic topical composition, including a polymer of high molecular weight, with a base of at least one monomer having a free strong acid group copolymerised with at least one esterified monomer and terminated by a hydrophobic group having 8 to 30 carbon atoms.
The abovementioned polymer is an emulsifying polymer, in solid form; it can be dispersed in water and it enables the composition which contains it to be thickened, notably for pH values close to 5.
However, these polymers have the disadvantages relating to the use of a powder: difficulties of transport and cleaning, dangerousness of the product relating to its powdery, irritant and particular character. In addition, these polymers must be solubilised in the medium to be thickened, through the introduction of surfactants. The latter constitute additional formulation additives which make the formulation more complex, and which can interact with the surfactants already contained in the formulation, sometimes creating undesirable effects (notably phase separation, or the formation of residual insolubles).
The technique called “back-acid” is also known, as described in document WO 01/76 552. This is a method consisting in introducing a surfactant and an alkali-swellable acrylic copolymer into an aqueous medium. This leads to a thickening effect when its carboxylic acid groups are neutralised: a three-dimensional network is then created which leads to an increase of the viscosity of the aqueous phase. Such an effect may be activated in a pH area close to 6, the role of the surfactant being to maintain the thickening effect, even when the pH is reduced.
In addition to the abovementioned ion mechanism, there is an associative mechanism, based on the presence of a hydrophobic monomer: this is what is described by document WO 03/62 288, which also seeks to thicken formulations with an acidic pH. The same goes for document U.S. Pat. No. 4,529,773 A1. As with the back-acid method, the presence of a surfactant in the form of an additional product is therefore necessary, leading to the previously mentioned disadvantages.
A number of documents which describe the use of other polymers in emulsion are also known. In this respect, document EP 0 824 914 B1 describes a polymer containing an aminated cationic monomer. The sought thickening effect will be obtained with an acidic pH through ionisation of the aminated cationic monomer. In document WO 2004/024 779, the cationicity of the envisaged polymer is provided by a substituted amino vinylic monomer. With this case also it is possible to thicken an aqueous medium with an acidic pH.
However, the toxicity of cationic polymers for aquatic fauna is well known: and they are unfortunately found at the life-cycle end in our rivers, both large and small, into which they are discharged through the domestic water system.
Finally, reverse emulsions and their applications as thickening agents in the cosmetics field are also known, as disclosed in documents WO 2004 063228 A1 and GB 2 422 605 A 1. However, these structures require the presence of surfactants and solvents to ensure their stability, and the disadvantages mentioned above are then posed.